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12V Relay DIY write up

Discussion in 'Modifications and Projects' started by rearviewmirror, Jun 11, 2010.

  1. I threw together a short write up on installing a 12V relay so devices turn off when the ignition is switched off. After having my GPS and heated grips hardwired to the battery for awhile I finally got fed with worrying that I might have left them on, or worse yet, some knob out on the street might turn on the heated grips. People love to push buttons! So this solves the worrying problem...


  2. I know the benefits of doing this, although electricity in general baffles me, but this is a good explanation, well documented (y).
  3. Thanks for this write up, it answers the question I was doing a search for (ie wiring up the relay without a distribution block). I just bought the below relay from Jaycar. $10 and it includes a fuse. Just two questions:

    -I don't believe I have an unused 12v connection for attaching to the earth tab 85. From what I understand from this link http://www.canyonchasers.net/shop/generic/relay.php , I ought to attach it to the rear light. So I splice it into to the positive wire going into the light?

    -The fuse provided is 30amp - I believe I ought to change to 20? And I assume a fuse in that position is adequate?


  4. Dig the fused relay, even though my grips have a fuse

    And yes, easiest to tap into the tail light wire
  5. Matt, looking at the diagram of that relay of yours, Pins 85 and 86 supply power to the coil or electromagnet. One goes to a supply that comes on and off with power. The other connects to earth.

    The supply can come from your taillight. Just be wary though - its a low power item. I'd suggest connecting to something that uses more power, say, the headlight circuit (if it comes on and off with the ignition, but doesn't have a separate headlight on/off switch).

    The other two pins are either side of the switch that turns the accessories on and off. One to the battery, the other to the accessories, or distribution rack or whatever.

    That 30a fuse should be adequate if you're firing up heated grips and other accessories. Otherwise look at what it's powering, total up their amp requirements and use that rated fuse.

    Just a word of advice. Ensure that the circuit that you tap into to power the coil is also protected by a fuse. Should the coil burn out and cause a short then you'll have no protection for that circuit and you could suffer a significant level of damage to the wiring, or worse.
  6. Good write up, is that an indicating LED on your inline fuse? Where did you get that fuse holder? Great way of seeing if your fuse has blown without the need of any test equipment
  7. nice work, but I'm not following this bit

    shouldn't the switched power go to relay #86 and not the earth tab (#85)?

    FWIW, I've put a relay in to feed more power to my coils (1980s loom)

  8. Yes, that is what is happening there. The drawings are all correct but the description is wrong. Good pickup, I only skimmed it before
  9. It can be connected either way. The coil isn't polarity sensitive.
  10. Maybe, but as described, it has two earth wires going to it. I think it may just be an error, because the diagram seems to be correct.

  11. Where are you seeing this, Bikeboy? I've had another look at the diagram and it's correct, schematically.
  12. yes, the diagram is correct, but the text refers to switched power connecting at pin #85 - which is an earth. The diagram shows it connecting to pin #86.

  13. Oh, OK, fairy nuff...
  14. I'll edit it. I wrote that in a hurry. I'm a EE by trade, you'd think I know better. Well good engineers build things, we're not so good at writing it down, that's why we have tech writers. :D

    EDIT... write-up has been corrected.
  15. The taillight circuit is more than adequate, the relay switch has a very low current draw, the current the taillight pull is irrelevant.